1. Field of the Invention
The present invention relates to an exhaust gas purifying system of an internal combustion engine. More specifically, the invention relates to an exhaust gas purifying system that includes a selective reduction catalyst for reducing NOx under the presence of NH3.
2. Related Art
Hitherto, as one of exhaust gas purifying systems that purifies NOx in an exhaust gas, a configuration is proposed in which an exhaust passageway is provided with a selective reduction catalyst that selectively reduces NOx in an exhaust gas by a reducing agent such as ammonia (NH3) (for example, see Patent Document 1). For example, in a urea addition type exhaust gas purifying system, urea water as a precursor of NH3 is supplied from the upstream side of a selective reduction catalyst, NH3 is produced from the urea water by heat degradation or hydrolytic degradation using the heat of the exhaust gas, and thus NOx in the exhaust gas is selectively reduced by the NH3. In addition to such a urea addition type system, for example, a system is also proposed which produces NH3 by heating a compound of NH3 such as ammonia carbide and directly adds NH3 to a subject.
In many cases, the selective reduction catalyst is used in an exhaust gas purifying system of an engine such as a lean combustion type gasoline engine or diesel engine operated based on lean combustion in which the air-fuel ratio of the air-fuel mixture is set to be leaner than stoichiometry so as to exhibit high NOx purification performance under the presence of an exhaust gas of a lean air-fuel ratio containing a large amount of oxygen. However, in an acceleration operation in which a NOx discharge amount increases, there is a case in which NOx may not be sufficiently purified only by the selective reduction catalyst. Therefore, as in the system disclosed in Patent Document 2, it is considered that NOx is purified by using a three-way purification reaction in a three-way catalyst provided at the upstream side of the selective reduction catalyst during the acceleration operation. According to the exhaust gas purifying system of Patent Document 2, in the system in which the three-way catalyst is provided at the upstream side of the NOx absorption-reduction catalyst, the air-fuel ratio of the air-fuel mixture is changed from the lean side to the stoichiometry in order to use the three-way purification reaction in the three-way catalyst during the acceleration operation.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2008-2065    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2009-293585